This invention relates generally to transducers and more particularly to transducers adapted to detect sonic energy.
As is known in the art, transducers have a wide range of applications as, for example, hydrophones used to detect sonic energy associated with underwater objects. One such type of sonic device known in the art uses the piezoelectric properties of a ceramic material whereby an electrical signal is produced in such ceramic material in response to mechanical stress and corresponding strains produced in the ceramic material in response to longitudinal pressure waves associated with the applied sonic energy. Another material suggested for such piezoelectric sonic device is polyvinyledene-fluoride (PVDF) polymer as described in an article entitled "Model for a Piezoelectric Flexural Plate Hydrophone" by Donald Ricketts, published in the Journal of the Acoustic Society of America, Volume 70, No. 4, October 1981. A sheet of such PVDF material is coated on opposite surfaces with electrical conductive layers. The coated sheet is then submerged in an ocean body to detect sounds emitted by, or reflected by, underwater objects. These sounds cause stresses and strains in the PVDF material. A voltage is produced across the conductive layers which is related to the piezoelectric characteristics of the PVDF material to thereby detect these sounds. Because low frequency (less than 100 Hz) sound travels over long distances under-water without excessive attenuation and can be heard at long ranges, it is desirable to have sonar transducers which can effectively detect sonic energy at these low frequencies. While the piezoelectric sonic devices described above are useful in many applications, the detection of low frequency signals, i.e., those sonic signals having frequencies below 100 Hz, becomes difficult with such devices. For example, use of the PVDF polymer piezoelectric device in detection of these low frequency sonic signals generally requires a relatively thick polymer thereby increasing the cost of such device. Further, the ceramic piezoelectric devices inherently have a relatively limited low frequency response characteristic. Further, the ceramic piezoelectric sonic device is sensitive to accelerations and vibrations which may occur as a result of the mounting of such a piezoelectric device to the hull of a ship, for example, thereby interfering with the effective sonic detection sensitivity of the device.